1. Field of the Invention
The present invention relates to a color filter for a liquid crystal display device having a good visibility from an oblique direction and from a vertical direction, and to a liquid crystal display device provided with the color filter.
2. Description of the Related Art
Liquid crystal display devices are those utilizing birefringence of a liquid crystal molecule and includes a liquid crystal cell, a polarizing element and an optical compensation layer. Liquid crystal devices like this are classified by the type of light source into two categories, that is, a transmission type device provided with a built-in light source and a reflection type device utilizing an external light source.
The transmission type liquid crystal display device has a structure in which two polarizing elements are arranged on both sides of the liquid crystal cell and one or two optical compensation layers are disposed between the liquid crystal cell and the polarizing element. Also, the reflection type liquid crystal display device has a structure in which a reflecting plate, a liquid crystal cell, one optical compensation layer and one polarizing element are arranged in this order.
In the liquid crystal cell, aligned rod-like liquid crystalline molecules are interposed between two substrates and a voltage is applied to an electrode layer disposed on one or both sides of two substrates to thereby vary the alignment state of the rod-like liquid crystal molecules, thereby switching on or off the transmission/shielding of light.
Liquid crystal cells having various display modes are proposed according to different alignment states of the rod-like liquid crystalline molecule. These liquid crystal cells includes TN (Twisted Nematic), IPS (In-Plane Switching), FFS (Fringe Field Switching), FLC (Ferroelectric Liquid Crystal), OCB (Optically Compensated Bend), STN (Supper Twisted Nematic), VA (Vertically Aligned) and HAN (Hybrid Aligned Nematic).
The polarizing element generally has a structure in which two transparent protective films including triacetyl cellulose (hereinafter referred to as “TAC”) are applied to each side of the polarizing film obtained by diffusing iodine in a polyvinyl alcohol (hereinafter referred to as “PVA”) and by stretching the film.
Various types are proposed as the compensation layer. For example, in a VA-mode liquid crystal display device having good display characteristics in a wide range of viewing angles, a biaxial retardation film having an index ellipsoid having the following relation between three-dimensional principal indices nx, ny and nz: nx≧ny>nz is combined (see, for example, JP-A 2007-328324).
A liquid crystal display device is valued for its space saving and light-weight characteristics owing to its thin form, and also because it saves power, thus has rapidly become widely used in televisions and other AV equipment and also, on the other hand, it is strongly desired that the liquid crystal display device is more developed in display performances such as luminance, contrast and omnidirectional visibility.
Specifically, normally black mode IPSs and VA liquid crystal display devices enabling higher contrast and wide-viewing angle display are particularly preferably used in television applications. Also, as the optical compensation layer mentioned above, those so-designed that coloring as viewed from the front when displaying black and a variation in color as viewed from the front are minimized are used.
However, most of the optical compensation layers to be used in the aforementioned VA mode liquid crystal display device are generally biaxial retardation films formed by biaxial alignment or retardation films formed by applying a polymerizable liquid crystalline and/or non-polymerizable liquid crystalline material. It is therefore difficult to produce these retardations films having three-dimensional refractive indices nx, ny and nz controlled at the high level of display quality required these days.
Specifically, it is necessary that the three-dimensional principal refractive indices be determined in consideration of not only the birefringence of the liquid crystal material but also even the retardation values (hereinafter, referred to as Rth (R), Rth (G) and Rth (B)) in the directions of the film thicknesses of the red, green and blue color pixel layers constituting the color filter prior to the production of the compensation layer. However, it is difficult to control the in-plane retardation values represented by two parameters nx and ny and the retardation values represented by three parameters nx, ny and nz in the thickness directions simultaneously with high accuracy and it is also difficult to provide the optical compensation layer with both the wavelength dispersibility compensating the birefringence of the liquid crystal material and the wavelength dispersibility compensating the retardation values in the thickness direction of each of the red, green and blue color pixel layers for light having wavelengths in red, green and blue regions. It cannot be said that current liquid crystal display devices are designed to have an optimum wavelength dispersibility.
As a result, though the visibility from the front (vertical direction) with respect to the display surface is better, optical compensation for visibility from an oblique direction at an angle of, for example, 45 degrees with respect to the front (vertical direction) (hereinafter simply abbreviated as “oblique visibility”) is not optimally achieved, and therefore, only a specific color light leak, resultantly causing reddish, bluish or greenish coloring when black is displayed.
A color filter has a relatively small retardation compared to other members used in a liquid crystal display device. Therefore, in a conventional system liquid crystal display device, the compensation ability of the optical compensation layer is designed with almost no consideration given to the retardation of the color filter. However, the retardation level has come to be non-negligible in liquid crystal televisions requiring a high contrast and wide viewing angle characteristics.
In, particularly, liquid crystal display devices having a contrast as high as 1000 or more and especially 3000 or more, there is a high demand for the image qualities of black display, and this becomes a problem to be solved.
On the other hand, there is an attempt to reduce the retardation of a color filter by formulating a polymer having a plane structural group at its side chain in a color polymer thin film or by formulating birefringence-reducing particles having a reverse positive or negative birefringence with respect to the color polymer thin film (see, for example, JP-A 2000-136253 and 2000-187114).
Also, a method is disclosed in which the in-plane retardation of the blue region of a color filter is made to be larger than those of the green region and red region to thereby increase the leakage of blue light, thereby offsetting yellowing which has a complementary color relation with blue all over the display surface, to make an improvement in the yellowing of the whole display surface when viewing the liquid crystal display device from an oblique direction (see, for example, JP-A 2001-242460).
Also, a method is disclosed to improve the oblique visibility, wherein the retardation values Rth (R), Rth (G) and Rth (B) in the thickness directions of red, green and blue pixels of a color filter are so designed as to satisfy the equation Rth (R)>Rth (G)>Rth (B) or Rth (R)<Rth (G)<Rth (B) in accordance with each wavelength dispersibility of a liquid crystal material and retardation film (see, for example, JP-A 2007-212603).
However, the inventors of the present invention have found that the retardation value in the thickness direction of a color filter largely differs depending on the type of pigment to be used and also, the degree of retardation value in the thickness direction of is increased by micronizing or dispersing the pigment or by using a matrix resin (for example, acryl resins and card resins). These methods using polymer thin films or formulating birefringence-reducing particles fail to obtain sufficient effects and therefore, cannot solve the above problems.
Particularly, a color filter using, as its base material, a transparent resin typified by acryl resins improved in the dispersibility of organic pigments for use in a high contrast liquid crystal display device has a difficulty in improving oblique visibility while maintaining a high contrast value (preferably 1000 or more and more preferably 3000 or more) required for the color filter.
Additionally, a color filter having a small birefringence is simply an excellent one in conventional technologies. Though studies as to measures for improving the oblique visibility have been made, almost no study has been made as to measures for adjusting the optimum value of the retardation in the thickness direction of each color of the color filter to the level at which no problem is posed on display of black state in consideration of the wavelength dispersibility of the birefringence of the liquid crystal material and optical compensation layer, as a high contrast liquid crystal display device.